Cells need to uptake amino acids regularly for nutrition, and this function is played by amino acid transporters, which are membrane proteins locating on the cell membrane. In particular, a neutral amino acid transport system L, which takes a part in supplying various essential amino acids to cells, is one of the most important transport mechanisms for cellular nutrition, and this system plays important roles in absorption from the intestine, resorption from the renal tubular, and passing through the blood tissue barrier as well. Further, the neutral aminoacid transport system L has been known to transport analogs of the neutral amino acids or drugs and toxic substances having similar structure to the neutral amino acid as well, due to wide selectivity for substrate.
The neutral amino acid transport system L has been originally first reported as an amino acid transport system in a cancer cell line being inhibited specifically by 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH) as an analogous compound to amino acid. Since then, the system L has been studied using cultured cells, specimens of membrane vesicles, specimens of removed organs, or in vivo specimens (Christensen, Physiological Reviews, Vol. 70, No. 1, 43-77 (1990)). The neutral amino acid transport system L is sodium-independent transporter, in other words, the transport system L does not require sodium ion for its function. It has been known that there are differences in selectivity for a substrate to be transported and transport characteristics depending on each cell and tissue.
However, by means of the conventional methods, it is difficult to analyze details of the transportation of the neutral amino acids and their analogs and the role of the neutral amino acid transport system L for viability or growth of cells. Enabling precise analysis of the function has been desired by isolating genes of the neutral amino acid transporter, which takes on the function of the neutral amino acid transport system L.
With regard to the neutral amino acid transporter, sodium-dependent transporters, ASCT 1 and ASCT 2, have been cloned (Kanai, Current Opinion in Cell Biology, Vol. 9, No. 4, 565-572 (1997)). However, these transporters, which work principally with alanine, serine, cysteine, threonine and glutamine as main substrates, are different from the neutral amino acid transport system L in substrate selectivity. Further, a glycine transporter and a proline transporter have been cloned, but these are also different from the neutral amino acid transport system L (Amara and Kuhar, Annual Review of Neuroscience, Vol. 16, 73-93 (1993)).
cDNAs of rBAT and 4F2hc have been cloned, which are not the transporters themselves, but considered to be activators of the amino acid transporters and are type-2 membrane glycoproteins having only a single transmembrane structure, and it is known that when these cDNAs are expressed in Xenopus oocyt uptake of basic amino acids as well as neutral amino acids have been activated (Palacin, The Journal of Experimental Biology, Vol. 196, 123-137 (1994)).
As a transporter corresponding to the transport system L, a neutral amino acid transporter LAT1 (Kanai et al., The Journal of Biological Chemistry, Vol. 273, No. 37, 23629-23632 (1999)) and LAT2 (Segawa et al., The Journal of Biological Chemistry, Vol. 274, No. 28, 19745-19751 (1999)) have been cloned. Both of them are transporters functioning by forming heterodimers with 4F2hc and exhibiting sodium-ion (Na+) independent transport. LAT1 shows an exchange transport activity for large neutral amino acids such as leucine, isoleucine, valine, phenylalanine, tyrosine, tryptophan, methionine and histidine, and LAT2 has wide substrate selectivity of transport not only for large neutral amino acids but for small neutral amino acids such as glycine, alanine, serine, cysteine, and threonine. However, the systemic transport system L cannot be explained only by these two types transporters of the transport system L, and therefore, existence of an unidentified isoform of transport system L has been expected.
The known transport system L transporters of LAT1 and LAT2 are heterodimeric proteins belonging to SLC7 family, and form functional transporters by coupling with 4F2hc which is a protein having single transmembrane structure. In the already disclosed mouse and human genomic database, a functionally unidentified member of SLC7 family was searched, but any additional transporter corresponding to the transport system L was not found. Therefore, unidentified new transport system L transporter has been supposed to be a protein other than the SLC7 family.
Further, as analogous proteins to the neutral amino acid transporter LAT1, y+LAT1 and y+LAT2, which have a function of transport system y+L to transport neutral and basic amino acids, have been cloned (Torrents et al., The Journal of Biological Chemistry, Vol. 273, No. 49, 32437-32445 (1998)). In addition, it was demonstrated that both of y+LAT1 and y+LAT2 functioned only in the coexistence with a complementary factor 4F2hc. Both of y+LAT1 and y+LAT2 principally transport glutamine, leucine and isoleucine as neutral amino acids, and have narrow substrate selectivity for the neutral amino acid.
In addition, as an aromatic amino acid transporter, TAT1, which corresponds to transport system T, has been cloned (Kim et al., The Journal of Biological Chemistry, Vol. 276, No. 20, 17221-17228 (2001)). The TAT1 transports aromatic amino acids such as tryptophan, tyrosine and phenylalanine Na+-independently, but does not transport blanched amino acids such as leucine, isoleucine and valine. TAT1 is not inhibited by BCH, which is a specific inhibitor of the transport system L, and thus, the TAT1 is distinct from the amino acid transport system L.
The references of the prior art with respect to the present application of the invention are as follows;    1. Christensen, Physiological Reviews, Vol. 70, No. 1, 43-77 (1990)    2. Kanai, Current Opinion in Cell Biology, Vol. 9, No. 4, 565-572 (1997)    3. Amara and Kuhar, Annual Review of Neuroscience, Vol. 16, 73-93 (1993)    4. Palacin, The Journal of Experimental Biology, Vol. 196, 123-137 (1994)    5. Kanai et al., The Journal of Biological Chemistry, Vol. 273, No. 37, 23629-23632 (1999)    6. Segawa et al., The Journal of Biological Chemistry, Vol. 274, No. 28, 19745-19751 (1999)    7. Torrents et al., The Journal of Biological Chemistry, Vol. 273, No. 49, 32437-32445 (1998)    8. Kim et al., The Journal of Biological Chemistry, Vol. 276, No. 20, 17221-17228 (2001)    9. Cole et al., Genomics, Vol. 51, No. 2, 282-287 (1998)